Zinc-coated steel sheet is being broadly used in the fields of construction, automobiles, etc. from the viewpoint of improvement of the corrosion resistance of structural members. For improvement of the corrosion resistance in conventional structures, the method has been used of welding together non-plated members, then dipping the assembly in a zinc bath to make zinc deposit on the surface of the steel material and weld zone and secure the corrosion resistance of the structure as a whole. However, with this method, since the plating is performed after welding, the productivity is inferior. Also, a plating bath and other facilities become necessary. This became a cause of increase of the manufacturing costs.
To avoid this, the method of producing a structure by welding together zinc-coated steel sheets which have been plated in advance has come to be applied. Further, recently, to better improve the corrosion resistance of structural members, the practice has been to produce welded structures by welding together zinc-based alloy coated steel sheets given a Zn—Al—Mg—Si-based alloy plating or other zinc-based alloy coating on the surface of the steel sheets so as to further raise the corrosion resistance compared with general zinc-coated steel sheet (for example, see PLT 1).
As a unique problem when producing a welded structure by welding together zinc-coated steel sheets or zinc-based alloy coated steel sheets, it has been known in the past that liquid metal embrittlement cracking due to hot dip plating (below, referred to as “zinc embrittlement cracking”) easily occurs in the weld metal and base metal heat affected zone.
Zinc embrittlement cracking is believed to be mainly due to the zinc plating components, which remain in a molten state at the surface of the base metal heat affected zone present near the weld zone, penetrating to the crystal grain boundaries at the weld zone. Note that, the zinc plating which was present at the surface of the weld zone is dispersed away by the welding, so it is believed that this does not become the cause of weld embrittlement cracking.
On the other hand, for the welding of stainless steel structures from which corrosion resistance was demanded in the past, an alloy welding material of stainless steel has been used. In this case, even with weld metal of stainless steel components which is formed at the joined parts of the stainless steel with itself or stainless steel and carbon steel, an excellent corrosion resistance is obtained in the same way as the base material part of stainless steel.
However, according to the results of confirmation tests by the inventors, to obtain a weld metal with a good corrosion resistance when welding zinc-coated steel sheet, for example, it was confirmed that even if using a SUS309-based or SUS329-based stainless steel welding material or other welding material, a large number of zinc embrittlement cracks occur at the weld metal and therefore application becomes difficult.
As the method for solving the problem of the zinc embrittlement cracking of weld metal, the inventors proposed to control the amounts of C, Si, Mn, Ni, and Cr obtain a suitable area percentage of ferrite structures in the weld metal and tensile strength and furthermore to control the amount of TiO2 in the slag forming agent etc. in flux-cored welding wire to suitable values to prevent zinc embrittlement cracking in the weld metal (see PLT 2).
However, when using this method to weld zinc-based alloy coated steel sheet, depending on the welding conditions, zinc embrittlement cracking of the weld metal often occurs. It was not possible to stably prevent this. Further, there were the problems that the welded metal obtained by this method was low in ductility and further the arc stability in weld work efficiency was low and the slag detachability was poor.
To deal with this, the inventors engaged in further in-depth research on welded joints preventing weld embrittlement cracking and proposed the means of defining the weld metal components of the joint so as to suppress zinc embrittlement cracking occurring at the weld metal (see PLT 3). Furthermore, in that PLT, they proposed the means of defining the alloy composition of the welding wire so as to adjust the weld metal components of the joint to targeted ranges.